The present invention relates in general to magneto-resistive random access memory (MRAM). More particularly, the present invention relates to a spin hall write selection apparatus and method for use with MRAM.
Data in MRAM is stored by magnetic storage elements. The elements are formed from ferromagnetic plates that each hold a magnetization and which are separated by a thin insulating layer. One of the two plates is provided as a permanent magnet, which is set to a particular polarity, and the magnetization of the other plate can be changed to match that of an external field in order to store data. This configuration is known as a magnetic tunnel junction (MTJ). A memory device can be built from a grid of MTJ “cells.”
Reading from an MRAM memory device can be accomplished by measurement of electrical resistance of a cell. A particular cell is typically selected by powering an associated transistor that switches current from a supply line through the cell to ground. Due to the tunnel magneto-resistance, the electrical resistance of the cell changes due to the relative orientation of the magnetization in the two plates. By measuring the resulting current, the resistance inside any particular cell can be determined and from this the magnetization polarity of the writable plate can be determined as well. If the two plates have the same magnetization alignment (low resistance state) this is usually considered to mean a “1” while if the alignment is antiparallel the resistance will be higher (high resistance state) and this usually means a “0.”
Data is written to the cells using a variety of methods. In one, each cell lies between a pair of write lines arranged at right angles to each other and in parallel to the cell, one above and one below the cell. When current is passed through them, an induced magnetic field is created at the junction, which the writable plate picks up. However, since this approach requires a fairly substantial current to generate the field, it is typically less applicable for low-power cases and is thus illustrative of one of MRAM's primary disadvantages. Additionally, as the device is scaled down in size, there comes a time when the induced field overlaps adjacent cells over a small area, leading to potential false writes. This problem, the half-select (or write disturb) problem, appeared to set a fairly large minimum size for this type of cell.
Newer techniques have therefore been proposed to avoid the above-described issues. One such technique relates to spin transfer torque (STT) or spin transfer switching and uses spin-aligned (“polarized”) electrons to directly apply torque to the domains.
Spin torque MRAM uses a 2-terminal device with a pinned or fixed layer that has a fixed magnetization polarity, a tunnel barrier and a free layer that has a free or variable magnetization polarity. The fixed layer, the tunnel barrier and the free layer are provided in a MTJ stack. The magnetization of the fixed layer is fixed in a given direction and a current passed up through the junction stack makes the magnetization of the free layer parallel with respect to that of the fixed layer whereas a current passed down through the junction stack makes the magnetization of the free layer anti-parallel with respect to that of the fixed layer. A relatively smaller current (of either polarity) is then used to read the resistance of the device which is dependent on the relative orientations of the free and fixed layers.
In previous applications of spin torque MRAM, the free and fixed layers have their magnetizations lie in-plane. However, this leads to the need for high switching currents. While materials with perpendicular or non in-plane magnetization can be used (i.e., perpendicular magnetic anisotropy or PMA), the switching currents are still higher than desired.